Wondering if the tolerances are the same between the new engines and say 10 years ago or more? Such as piston wall clearances and bearing clearances.
To me it seems you can only go so close before you would run into troubles.
Ken, there is a complex formula concerning the clearance between the journal and bearings.
Here is a nice article from Molakule that touches it.
"Bearing friction is determined by an empirical equation (derived by test data) called the “McKee” equation as: Eqn. 1, f = 1x10^-10[473(ZN/p)D/C] + k, where k is a constant determined from another equation or chart for various L/D ratios; L is length of bearing (in.), D is diameter of bearing (in.), N is speed of journal in rpm, N’ is speed of journal in rps, Z is absolute viscosity in cP, C is diametral clearance between bearing and journal in inches. For most bearings in automotive use, k = 0.002. p is pressure = W/LD, W is bearing load in lbs. An average value for D/C is 1000. In automobile and piston aircraft engines, the loads W, are: Main – 700-1700 lbs., Crankpin – 1,400-3,400 lbs., and Wrist pin - 2,000-5,000 lbs. The ratio of e to the radial clearance is called the “attitude” and is defined as: Eqn. 2, A = 2e/C = 1- 2Ho/C. Ho is the “Minimum Film Thickness.” So how does one find the viscosity required for a bearing? One uses the “McKee” equation, along with the “Lasche” equation, for bearing heat generation and dissipation, Hd and Hg, and equates these equations to find the viscosity required!"
Ok let me rephrase that
Has Ford and Honda tightened their clearances compared to what was in use before 5W/20 oil was recommended. My old Toyota a 86 Corolla is speced at when new of course
Piston to bore clearance 0.0039-0.0047 in
Connecting rod clearance
0.0008 to 0.0020 in
Crankshaft clearance 0.0006-0.0013 in
Or has oils improved enough to be able to use thiner oils now.
This topic came up on a Ford board about Ford now saying 5w20 is backwards compatible to older vehicles that were using 5w30. IMO so the service department doesn't need to carry more than 1 kind of bulk oil. That tells me tolerances have not changed.
5w20 may have proven itself to be good, but the reasons still stink, 0.6% increase in mileage. On a vehicle that gets 20mpg thats an increase of a whopping 0.12
It's obvious that most engine can run 5w-20 weight all the time. If they've been using 5w-30 for the recommended OCI, they've been running on it anyway due to shearing. Since this use of shearing 30 weight hasn't seemed to have destroyed any engines when used according to manufacturers recommendations ..the use of a shear stable 5w-20 from the get go should have no effect on them either. Now if an engine was spec'd for a 10w-30, then it's inferred that this typical shearing is not desired and that the need for a 30 weight takes precedence.
Since 5w-30 has been in common usage for well over a decade without difficulty, we can assume that all clearances in place over that time span can accept a 5w-20 where 5w-30 was spec'd over an OCI of a duration that inherent shearing would have occured.